It depends, but I think the value of forcing a junior to sue an OS they don't want to use is minimal. This is especially so when you are trying to force them to use what is probably a minority OS that is not used by anyone else in the organisation apart from the unix admins.
I personally think it is more convenient for doing actual unix work to have a unix workstation.
It means you can easily develop scripts on your desktop host.
It also lets you try stuff.
That's my personal preference.
But I don't think it is essential.
Running a linux desktop is not the same as running a linux server and while a lot of what the young admin may learn maybe useful in his server work, a lot of it, will not. E.g. setting up X11 servers is not very useful on most unix servers.
He'll get the server relevant experience on the servers.
The workstation specific stuff he learns by running a unix workstation for the most part won't really help him be a better server admin.
On the other side if you work in an environment where most of the business tools (e.g. office, visio, OCS) are only available on windows or the unix versions of them are sub standard (which I think is probably true for most environments these days), and you only get one workstation, then using windows as your primary workstation may not be such a bad thing, especially if you need to use those business tools a lot (e.g.e if policy douments and work requests come as word and excel docs).
It is after all still perfectly possible to do all of your work on unix with putty, winscp and if required a windows X11 server.
I've worked in environments where you simply weren't allowed to use a non windows OS for your workstation.
I managed perfectly well with putty and winscp.
I was occasionally frustrated with a windows workstation, and I moaned and complained bitterly when I first learnt that I was forced to use windows, but as it turned out the frustration occurred far less than I thought it would when I started.
We did however have a unix host that we could use for things like copying large logs to for processing, script dev, running LDAP commands etc.
In addition sometimes I think it can be less productive using a unix desktop.
E.g. it can be a real pain to get things like flash and java plugins working on a linux workstation and these things are at times required for tools like Storage GUIs whcih run in web browsers. In some cases they pretty much will not run on browseers other than IE (e.g. it took me ages to get DRAC to work with firefox, and when I did I needed to run firefox as root - whereas it just worked with IE).
Also a windows desktop is useful in that it lets you experience and identify windows related problems with the unix services you provide much earlier.
Again ideally you'll have a test suite to identify these problems, but you'll identify them earlier probably before they even get close to the formal testing stage if you are using a windows workstation.
I think the only thing that would make me want to seriously consider making the junior use a unix workstation against his will is if the standard desktop OS in the organisation was unix. Then I think it would be reasonable for him to be required to use the standard desktop OS. The reasoning behind this is again so that they experience the services they provide the way other users in the organisation experience them.
I think that if the junior is going to grow up unixy, then he may well gravitate to a unix desktop over time. And if not I don't think it really matters as long as he can do his job efficiently.
My personal preference for a unix admin workstation is a Mac.
You get OFfice, exchange compatability if required, and you can use OCS.
You can't use an Access DB or visio, but you can RDP to a terminal server if these apps are required. And you have all the normal unix tools available - most of the common ones ship as standard and the others can be compiled or in most cases downloaded as mac installers.